Vibrating fork type field device with coil arrangement to induce vibration

Abstract

A field device comprising first and second tines; first and second rods coupled to the first and second tines; a coil arrangement fixed to the first rod; a magnet fixed to the second rod, opposite the coil arrangement; excitation circuitry coupled to the coil arrangement and controllable to provide a time-varying current to the coil arrangement, resulting in vibration of the first tine in relation to the second tine; sensing circuitry coupled to the coil arrangement and configured to provide a sensing signal indicative of a change in at least one property of the vibration of the first tine in relation to the second tine; and measurement control circuitry coupled to the excitation circuitry and the sensing circuitry for controlling operation of the excitation circuitry.

What is claimed is: 
     
         1 . A field device for determining a change in a property of a medium, the field device comprising:
 first and second tines to be arranged in the medium;   first and second rods coupled to the first and second tines, in such a way that vibration of the first rod in relation to the second rod results in vibration of the first tine in relation to the second tine;   a coil arrangement fixed to the first rod;   a magnet fixed to the second rod, opposite the coil arrangement;   excitation circuitry coupled to the coil arrangement and controllable to provide a time-varying current to the coil arrangement, resulting in vibration of the first tine in relation to the second tine;   sensing circuitry coupled to the coil arrangement and configured to provide a sensing signal indicative of a change in at least one property of the vibration of the first tine in relation to the second tine; and   measurement control circuitry coupled to the excitation circuitry and the sensing circuitry, and configured to:   control the excitation circuitry to provide the time-varying current to the coil arrangement;   receive the sensing signal from the sensing circuitry; and   determine the change in the property of the medium based on the change in the at least one property of the vibration and a predetermined relation between the change in the at least one property of the vibration, and the change in the property of the medium, wherein:   the field device comprises a housing;   the first and second rods, the excitation circuitry, and the sensing circuitry are enclosed by the housing; and   the first and second tines are arranged outside the housing,   wherein the excitation circuitry is fixed to the housing and coupled to the coil arrangement by wires.   
     
     
         2 . The field device according to  claim 1 , wherein the first and second tines and the first and second rods are arranged in such a way that, in an intersection of the field device with a given plane, the first tine is opposite and parallel with the second tine and the first rod is opposite and parallel with the second rod. 
     
     
         3 . The field device according to  claim 2 , wherein the coil arrangement and the magnet are intersected by the given plane. 
     
     
         4 . The field device according to  claim 2 , wherein the given plane is a symmetry plane for the first tine and for the second tine. 
     
     
         5 . The field device according to  claim 4 , wherein the plane is a symmetry plane for the coil arrangement fixed to the first rod and for the magnet fixed to the second rod. 
     
     
         6 . The field device according to  claim 1 , wherein the magnet fixed to the second rod is an electromagnet. 
     
     
         7 . A field device for determining a change in a property of a medium, the field device comprising:
 first and second tines to be arranged in the medium;   first and second rods coupled to the first and second tines, in such a way that vibration of the first rod in relation to the second rod results in vibration of the first tine in relation to the second tine;   a coil arrangement fixed to the first rod;   a magnet fixed to the second rod, opposite the coil arrangement;   excitation circuitry coupled to the coil arrangement and controllable to provide a time-varying current to the coil arrangement, resulting in vibration of the first tine in relation to the second tine;   sensing circuitry coupled to the coil arrangement and configured to provide a sensing signal indicative of a change in at least one property of the vibration of the first tine in relation to the second tine; and   measurement control circuitry coupled to the excitation circuitry and the sensing circuitry, and configured to:   control the excitation circuitry to provide the time-varying current to the coil arrangement;   receive the sensing signal from the sensing circuitry; and   determine the change in the property of the medium based on the change in the at least one property of the vibration and a predetermined relation between the change in the at least one property of the vibration, and the change in the property of the medium, wherein:   the field device comprises a housing;   the first and second rods, the excitation circuitry, and the sensing circuitry are enclosed by the housing; and   the first and second tines are arranged outside the housing, wherein:   the housing comprises a membrane; and   the first and second rods are coupled to the first and second tines via the membrane.   
     
     
         8 . A field device for determining a change in a property of a medium, the field device comprising:
 first and second tines to be arranged in the medium;   first and second rods coupled to the first and second tines, in such a way that vibration of the first rod in relation to the second rod results in vibration of the first tine in relation to the second tine;   a coil arrangement fixed to the first rod;   a magnet fixed to the second rod, opposite the coil arrangement;   excitation circuitry coupled to the coil arrangement and controllable to provide a time-varying current to the coil arrangement, resulting in vibration of the first tine in relation to the second tine;   sensing circuitry coupled to the coil arrangement and configured to provide a sensing signal indicative of a change in at least one property of the vibration of the first tine in relation to the second tine; and   measurement control circuitry coupled to the excitation circuitry and the sensing circuitry, and configured to:   control the excitation circuitry to provide the time-varying current to the coil arrangement;   receive the sensing signal from the sensing circuitry; and   determine the change in the property of the medium based on the change in the at least one property of the vibration and a predetermined relation between the change in the at least one property of the vibration, and the change in the property of the medium, wherein:   the coil arrangement comprises a first coil fixed to the first rod and a second coil fixed to the first rod;   the excitation circuitry is coupled to the first coil; and   the sensing circuitry is coupled to the second coil.   
     
     
         9 . The field device according to  claim 8 , wherein the first coil and the second coil are fixed to the first rod at different distances from the first tine. 
     
     
         10 . A field device for determining a change in a property of a medium, the field device comprising:
 first and second tines to be arranged in the medium;   first and second rods coupled to the first and second tines, in such a way that vibration of the first rod in relation to the second rod results in vibration of the first tine in relation to the second tine;   a coil arrangement fixed to the first rod;   a magnet fixed to the second rod, opposite the coil arrangement;   excitation circuitry coupled to the coil arrangement and controllable to provide a time-varying current to the coil arrangement, resulting in vibration of the first tine in relation to the second tine;   sensing circuitry coupled to the coil arrangement and configured to provide a sensing signal indicative of a change in at least one property of the vibration of the first tine in relation to the second tine; and   measurement control circuitry coupled to the excitation circuitry and the sensing circuitry, and configured to:   control the excitation circuitry to provide the time-varying current to the coil arrangement;   receive the sensing signal from the sensing circuitry; and   determine the change in the property of the medium based on the change in the at least one property of the vibration and a predetermined relation between the change in the at least one property of the vibration, and the change in the property of the medium, wherein:   the coil arrangement comprises a single coil fixed to the first rod;   the excitation circuitry is coupled to the single coil and configured to provide a multi-frequency excitation signal to the single coil; and   the sensing circuitry is coupled to the single coil and configured to sense a frequency-dependent impedance of the single coil.   
     
     
         11 . The field device according to  claim 10 , wherein:
 the excitation circuitry is coupled to provide the multi-frequency excitation signal as a multi-frequency voltage across a series coupling arrangement of the single coil and an auxiliary electric circuit element having a known impedance within a frequency range of the multi-frequency voltage; and   the sensing circuitry is coupled to sense the frequency-dependent impedance of the coil by sensing a voltage across the the single coil.   
     
     
         12 . The field device according to  claim 11 , wherein the excitation circuitry, the single coil, the auxiliary electric circuit element, and the sensing circuitry are coupled in a voltage divider configuration. 
     
     
         13 . The field device according to  claim 10 , wherein the multi-frequency excitation circuitry output voltage is a pulse, and/or a bandlimited noise signal, and/or a frequency sweep.